A lithium secondary battery (i.e., non-aqueous electrolyte secondary battery) that utilizes an organic lithium salt electrolyte has a reduced weight and a high energy density, and it has been expected to be a power supply for small electronic instruments, a small portable power supply, a power storage battery, and the like. Lithium metal has been used as a negative electrode material for lithium secondary batteries. Lithium metal is dissolved in an electrolyte as lithium ion during discharge, and lithium ion deposits on the surface of the negative electrode as lithium metal during charge. However, it is difficult to recover the original state of lithium ion deposited on the electrode surface (i.e., lithium ion tends to deposit in the form of a dendrite). Since the dendrite decomposes the electrolyte due to very high activity, the battery performance deteriorates, so that the charge-discharge cycle life decreases. Moreover, the dendrite may grow to reach the positive electrode, so that the electrodes may be short-circuited.
In order to overcome the above-mentioned drawbacks, use of a carbon material instead of lithium metal has been proposed. Since the carbon material does not allow lithium ion to deposit in the form of dendrite during storage or release of lithium ion, the carbon material is suitable for the negative electrode material. Specifically, a graphite material, such as natural graphite, has a high lithium ion storage/release capability, so that the storage/release reaction occurs promptly. Therefore, a graphite material ensures a high charge-discharge efficiency, and its discharge capacity is as high as the theoretical value of 372 mAh/g, and a high-voltage battery can be produced due to its low potential, which is almost equal to that of lithium metal during charge and discharge, for example.
A graphite material highly graphitized and having highly developed hexagonal carbon structures has a large discharge capacity, and it ensures a high first cycle charge-discharge efficiency of 90% or more. On the other hand, such a graphite material has a flat potential curve property during discharge, so that it is difficult to determine the discharge end point. Moreover, since a large amount of current cannot be discharged in a short time when a graphite material is used, the rate performance deteriorates, for example.
Various attempts have been made to solve the above-mentioned problems, such as improving the properties of a carbon material such as a graphite material (e.g., a carbon material having a two-layer structure obtained by covering the surface of a graphite material having a high degree of graphitization with a carbonaceous substance having a low degree of graphitization), or combining a graphite material having a high degree of graphitization with a carbonaceous substance having a low degree of graphitization, and so on.
For example, JP-A-4-368778 discloses a carbon negative electrode for secondary batteries obtained by covering the surface of carbon (active material) that comes in contact with an electrolyte with an amorphous carbon having a turbostratic structure, an average interlayer spacing in the C-axis direction of 0.337 to 0.360 nm, and a peak intensity ratio of 1360 cm−1 to 1580 cm−1 in an argon laser Raman spectrum of 0.4 to 1.0.
JP-A-6-267531 discloses an electrode material having a multilayer structure obtained by mixing particles of a carbonaceous substance (A) that satisfies the following condition (1) with particles of an organic compound (B) that satisfies the following condition (2), and the organic compound (B) is carbonized by heat treatment, so that the particles of the carbonaceous substance (A) are covered with a carbonaceous substance (C) that satisfies the following condition (3).
(1) The average lattice spacing d002 determined by wide-angle X-ray diffraction analysis is 3.37 angstroms or less, the true density is 2.10 g/cm3 or more, and the volume average particle diameter is 5 μm or more.
(2) The volume average particle diameter is smaller than that of the carbonaceous substance (A).
(3) The average lattice spacing d002 determined by wide-angle X-ray diffraction analysis is 3.38 angstroms or more, a peak PA exists in the range of 1580 to 1620 cm−1 and a peak PB exists in the range of 1350 to 1370 cm−1 when determined by Raman spectrum analysis using argon ion laser light, and the ratio R (IB/IA) of the intensity IB at the peak PB to the intensity IA at the peak PA is 0.2 or more.
These electrode materials are produced by forming an amorphous carbon layer on the surface of graphite particle. However, since graphitization easily progresses, it is difficult to form a thick amorphous carbon layer, so that the rate performance is not sufficiently improved.
A method to cover the surface of graphite particle with carbon black (amorphous carbon) has also been proposed. For example, JP-A-6-267533 discloses a lithium secondary battery wherein carbon black that has a DBP absorption of 100 ml/100 g or more and arithmetic mean primary particle diameter of 40 nm or more and carries lithium (negative electrode active material) is used as a negative electrode. Ethylene carbonate or an organic solvent that contains ethylene carbonate in an amount of 20 vol % or more is used as an electrolyte.
JP-A-7-153447 discloses a lithium secondary battery negative electrode that includes a carbon material that can store lithium (negative electrode active material) and a binder, wherein the carbon material is carbon black having a DBP absorption of 100 ml/100 g or more, and the binder is polyvinylidene fluoride.
JP-A-2001-332263 discloses a lithium ion secondary battery wherein the negative electrode includes graphite of which Gs=Hsg/Hsd is 10 or less in a surface-enhanced Raman spectrum. JP-A-2001-332263 also discloses a process of producing a carbon negative electrode material that includes mixing at least one of a carbon material and mesocarbon micro-beads grown at a temperature equal to or more than the production temperature and equal to or less than 2000° C., with a coating material that contains at least one of pitch containing free carbon, pitch having a quinoline-insoluble content of 2% or more, and a polymer, and graphitizing the material.
According to the above-mentioned negative electrode material, since the production temperature of the carbon material is low, the carbon material has an average lattice spacing d(004) of 0.336 nm or more. As a result, the reversible capacity decreases due to an insufficient degree of graphitization. Moreover, the above-mentioned documents do not aim at positively utilizing the high rate performance of carbon black. Moreover, when the surface of graphite is covered with pitch in which carbon black is dispersed, since the carbon black that is completely covered with the pitch has strong aggregating properties, the surface of the graphite cannot be covered with the pitch in which the carbon black is uniformly dispersed.